This Invention relates to a method of producing magnesium granules coated with a thin protective layer of fluoride, oxyfluoride or hydroxyfluoride.
Desulfurization of molten metal by magnesium is well known, for example, in the steel manufacturing process. Generally, magnesium is introduced into the molten hot metal by any of a variety of means, for example, a desulfurization agent may be added during steel manufacturing through injection lances. Upon injection, the magnesium melts, then vaporizes, and then the vapors dissolve into the molten hot metal. The dissolved magnesium reacts with sulfur present in the molten hot metal and forms magnesium sulfide, an insoluble compound. This insoluble compound has a density less than the density of molten hot metal and, thus, floats to the top of the molten metal and then mixes with the slag that is also floating on top of the liquid metal. The sulfur is removed by removal of the slag layer in a subsequent step.
It is also well known that magnesium when injected into steel causes a violent reaction, for example, when the addition of magnesium in particulate form, the violent reaction may take place in the form of bubbling, splattering, or the like. Also, finely ground particulate dust is difficult to meter in blast furnace injection processes and such finely ground dust injectables create a hazard in handling. For example, finely ground particulate when exposed to high temperature and oxygen may produce an explosion. Such may be the case with handling finely ground magnesium granules injected into a molten hot metal normally at process temperatures of up to about 1800 degrees centigrade.
Other known magnesium based injectable materials used as desulfurization agents include magnesium granules with a surface coating of a second material. For example, U.S. Pat. No. 4,331,711 discloses magnesium or alloys of magnesium particles coated with a salt such as a halide of Na, K, Li, Mg, Ca, Ba, Mn or Sr or mixtures of these salts.
U.S. Pat. No. 4,398,947 discloses a desulfurization agent including magnesium granules containing a coating of an anti-caking agent consisting of stearates of Mg, Ca and Al.
U.S. Pat. No. 4,401,465 discloses a desulfurization agent including substantially nonhygroscopic flux coated magnesium granules containing a coating of fluoride-containing salt such as alkali and alkaline earth metal fluorides and fluorborates.
Mixtures of powders of materials such as magnesium and calcium or lime are also known desulfurization agents. In addition, U.S. Pat. No. 4,705,561 discloses a composite material of a magnesium and calcium oxide useful as a desulfurization agent.
Another useful material as a desulfurization agent is disclosed in U.S. Pat. No. 4,708,737 which includes a magnesium impregnated with a magnesium alkaline earth metal compound and/or aluminum compound.
It is known to produce magnesium pellets by crushing and grinding magnesium ingots into various sizes and then screening the sizes to the desired size needed for a particular use.
For example, magnesium powder or granules are known to be used as a desulfurizing agent for a steel making process.
It would be desirable to provide a magnesium-based hot metal desulfurization reagent which is a single reagent. By "single reagent" it is meant that the reagent is not a mixture or composite material. A mixture can be, for example, a mixture of a magnesium and lime. It has been found that mixtures tend to separate into their individual components and, thus, there is an increased loss of metal with slag associated with the separation. The slag also creates a disposal or environmental problem. The production of a mixture increases the complexity of metering the appropriate proportions of blends of the components which is not always accurate or uniform.
It is also desirable to provide a magnesium based hot metal desulfurization reagent which is easy and safe to handle, has a low injection violence and has a high desulfurization efficiency.
It is desired to provide a magnesium-based powder or granule in solid or bulk form for use as a desulfurizing agent and possibly as a material which can be fed into extrusion presses or other forms of fabrication equipment without external remelt and its attendant losses. The granules may also be used as a material for alloying with other metals such as aluminum in conventional alloying processes. In addition, it is desired to produce a magnesium-coated desulfurizing agent which is safe to handle.